Techniques for metal gate work function engineering to enable multiple threshold voltage nanowire FET devices

ABSTRACT

A nanowire FET device includes a SOI wafer having a SOI layer over a BOX, and a plurality of nanowires and pads patterned in the SOI layer, wherein the nanowires are suspended over the BOX; an interfacial oxide surrounding each of the nanowires; and at least one gate stack surrounding each of the nanowires, the gate stack having (i) a conformal gate dielectric present on the interfacial oxide (ii) a conformal first gate material on the conformal gate dielectric (iii) a work function setting material on the conformal first gate material, and (iv) a second gate material on the work function setting material. A volume of the conformal first gate material and/or a volume of the work function setting material in the gate stack are/is proportional to a pitch of the nanowires.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. application Ser. No.13/588,724 filed on Aug. 17, 2012, the disclosure of which isincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to nanowire field-effect transistor (FET)devices, and more particularly, to techniques for gate work functionengineering using a work function setting material an amount of which isprovided proportional to nanowire pitch so as to enable multiplethreshold voltage (Vt) devices.

BACKGROUND OF THE INVENTION

In current complementary metal-oxide semiconductor (CMOS) scaling, theuse of undoped gate all around (GAA) nanowire devices is a highlyinvestigated structure as a device choice for future CMOS. One keyproblem with undoped devices is the implementation of multiple thresholdvoltage (Vt) devices. One solution is to dope the nanowire FET. To doso, however, for aggressively scaled devices has serious drawbacks fromrandom dopant fluctuation (RDF) effects and becomes extremelyproblematic as the nanowire diameter is scaled. One can also engineergate stacks with different work functions for different Vt's. Thishowever requires a substantial amount of process complexity.

Therefore, improved techniques for fabricating multiple Vt nanowire FETdevices that avoid the above-described drawbacks would be desirable.

SUMMARY OF THE INVENTION

The present invention provides techniques for gate work functionengineering in nanowire field-effect transistor (FET) devices using awork function setting material an amount of which is providedproportional to nanowire pitch. In one aspect of the invention, a methodof fabricating a nanowire FET device is provided. The method includesthe following steps. A semiconductor-on-insulator (SOI) wafer isprovided having a SOT layer over a buried oxide (BOX). Nanowires andpads are etched in the SOI layer, wherein the pads are attached atopposite ends of the nanowires in a ladder-like configuration. Thenanowires are suspended over the BOX. An interfacial oxide is formedsurrounding each of the nanowires. A conformal gate dielectric isdeposited on the interfacial oxide, surrounding each of the nanowires. Aconformal first gate material is deposited on the conformal gatedielectric, surrounding each of the nanowires. A work function settingmaterial is deposited on the conformal first gate material, at leastpartially surrounding the nanowires. A second gate material is depositedon the work function setting material, surrounding each of the nanowiresto form at least one gate stack over the nanowires. A volume of theconformal first gate material and/or a volume of the work functionsetting material in the gate stack are/is proportional to a pitch of thenanowires.

In another aspect of the invention, a nanowire FET device is provided.The nanowire FET device includes a SOI wafer having a SOI layer over aBOX, and a plurality of nanowires and pads patterned in the SOT layerwherein the pads are attached at opposite ends of the nanowires in aladder-like configuration, and wherein the nanowires are suspended overthe BOX; an interfacial oxide surrounding each of the nanowires; and atleast one gate stack surrounding each of the nanowires, the gate stackhaving (i) a conformal gate dielectric present on the interfacial oxide,surrounding each of the nanowires (ii) a conformal first gate materialon the conformal gate dielectric, surrounding each of the nanowires(iii) a work function setting material on the conformal first gatematerial, at least partially surrounding each of the nanowires, and (iv)a second gate material on the work function setting material,surrounding each of the nanowires. A volume of the conformal first gatematerial and/or a volume of the work function setting material in thegate stack are/is proportional to a pitch of the nanowires.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top-down diagram illustrating a plurality of nanowires andpads having been patterned in a semiconductor-on-insulator (SOI) layerover a buried oxide (BOX) according to an embodiment of the presentinvention;

FIG. 2 is a cross-sectional diagram illustrating an interfacial oxidehaving been formed around the nanowires and conformal gate dielectrichaving been deposited over the interfacial oxide according to anembodiment of the present invention;

FIG. 3 is a cross-sectional diagram illustrating a conformal first gatematerial having been deposited on the gate dielectric according to anembodiment of the present invention;

FIG. 4 is a cross-sectional diagram illustrating a work function settingmaterial having been deposited on the first gate material according toan embodiment of the present invention;

FIG. 5 is a cross-sectional diagram illustrating work function settingmaterial having been deposited on the first gate material using anangled deposition process according to an embodiment of the presentinvention;

FIG. 6 is a cross-sectional diagram illustrating a complete gate stackhaving been deposited onto the structure, surrounding the nanowiresaccording to an embodiment of the present invention;

FIG. 7 is a three-dimensional diagram illustrating gate stacks havingbeen patterned into gate lines surrounding the nanowires in a gate allaround configuration according to an embodiment of the presentinvention;

FIG. 8 is a three-dimensional diagram illustrating spacers having beenformed on opposite sides of the gate stack according to an embodiment ofthe present invention;

FIG. 9 is a three-dimensional diagram illustrating selective epitaxialgrowth having been used to thicken the exposed portions of the nanowiresand pads according to an embodiment of the present invention; and

FIG. 10 is a three-dimensional diagram illustrating a contact materialhaving been formed on the exposed epitaxial material according to anembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As described above, there are notable disadvantages associated withusing doping and/or different work function gate stacks to producemultiple threshold voltage (Vt) nanowire field-effect transistor (FET)devices. Advantageously, provided herein are techniques for producingmultiple Vt nanowire FET devices using a work function setting materialin an amount that is modulated as a function of nanowire pitch (wire towire pitch, where the pitch is defined as the distance from the centerof one nanowire to the adjacent nanowire(s)). Namely, a thickness of thematerials in the device gate stacks will be chosen such that less workfunction setting material ends up in the tighter pitch nanowire FETs.Thus, for smaller pitch, higher nanowire FET Vt is obtained andtherefore, through nanowire pitch variation, different Vt devices may befabricated. The technique does come at the cost of significant reductionin active width density, however if the lower Vt (wider pitch) devicesare not used for a large fraction of the chip area, and this trade-offmay be very preferred over the use of more complex (and yieldchallenging) integration schemes.

The present techniques are applicable in both gate-first and gate-lastnanowire FET process flows in which a gate all around (GAA)configuration is employed. In general, a gate-first approach to nanowireFET device fabrication involves patterning one or more nanowirechannels, releasing the nanowire channels from an underlying substrate,and forming a gate stack surrounding the nanowire channels. See, forexample, U.S. Pat. No. 7,884,004 issued to Bangsaruntip et al., entitled“Maskless Process for Suspending and Thinning Nanowires” (hereinafter“U.S. Pat. No. 7,884,004”), the contents of which are incorporated byreference herein. A couple of different gate-last approaches have beenproposed which vary depending on at what stage in the process thenanowires are formed. Accordingly, they are termed wire-first andwire-last approaches. In a gate-last wire-first approach, the nanowiresare first formed, a dummy gate is then formed on the wires. The dummygate is removed near the end of the process to be replaced by a metalgate stack. A gate-last, wire-last approach was developed to improve theprecision of the nanowire patterning process so as to permit moreuniform nanowires to be produced. See, for example, U.S. Pat. No.8,084,308 issued to Chang et al., entitled “Single Gate InverterNanowire Mesh” (hereinafter “Chang”) the contents of which areincorporated by reference herein. With a gate-last wire-last approach,precise patterning of the nanowires occurs after removal of the dummygate.

With either gate-last approach, a dummy gate is formed early in theprocess and then is removed and replaced with a replacement gate.Advantageously, the present techniques are easily integrated in either agate-first or a gate-last approach where the gate is formed over asilicon wafer.

The present techniques will be described by way of reference to FIGS.1-10. The process illustrated is representative of the steps that may beperformed after removal of the dummy gate in a gate-last wire-firstapproach or after nanowire patterning in a gate-first or gate-lastwire-last approach (see above). Namely, in all three cases, the presentprocess involves the fabrication of gate stacks surrounding a pluralityof nanowires to fabricate a nanowire FET device(s). With a wire-lastapproach, a dummy gate(s) may be used to locate the fin patterninghardmask relative to the source and drain regions of the device. See,for example, Chang. However, once the dummy gate(s) have been removed,the process steps for fabricating the gate stack (also called the“replacement gate” in the gate-last approach) are the same regardless ofwhether a gate-first or gate-last approach is being implemented.

Thus, the present process description begins with a plurality ofnanowires having been patterned on a wafer. In an illustrative example,the nanowires are patterned in a semiconductor-on-insulator (SOI) waferwith pads attached at opposite ends of the nanowires in a ladder-likeconfiguration (i.e., wherein the nanowires resemble the rungs of aladder). See FIG. 1. A portion of the nanowires which will be surroundedby the gate stack will serve as channels of the device(s). Thoseportions of the nanowires and pads extending out from the gate willserve as source and drain regions of the device. FIG. 1 provides atop-down view of these patterned nanowires and pads.

In the exemplary embodiment depicted and described below, multiplenanowire FET devices will be fabricated on the wafer (each device beingformed with a different nanowire pitch). For purposes of illustratingthe present techniques, two nanowire FET devices will be produced,namely a wide pitch nanowire FET and a tight pitch nanowire FET. By wayof example only, a tight nanowire pitch may be from about 20 nanometers(nm) to about 40 nm, whereas a wide pitch may be from about 40 nm toabout 80 nm. Of course, this configuration of devices is merelyexemplary and any other combination of devices, or even a single device,may be obtained using the present techniques. As will be described indetail below, a work function setting material(s) will be used in thegate stacks of the devices. The work function setting material acts as adoping source, and by way of the present process serves to change thework function of the gate stacks. Since the work function settingmaterial acts as a doping source, advantageously, the present processflow permits the same gate material (e.g., metal(s)) to be used in eachof the devices being formed (which simplifies the fabrication process).A different work function setting material can then be employeddepending, e.g., on whether an n-channel nanowire FET (NFET) or ap-channel nanowire FET (PFET) is desired. Further, metal from the gatestack will diffuse into the surrounding dielectric or gate material tochange the threshold voltage of the device. In one exemplary embodiment,the more work function setting material present in the gate, the lowerthe threshold voltage (V_(T)) of the device would be. Thus, by adding awork function setting material to the gate stack, the threshold voltagesof the resulting devices can be lowered. By way of the presenttechniques, more of the work function setting material will be depositedin the wide pitch devices as compared to the tight pitch devices. Bymodulating the volume of work function setting material proportionallyto the nanowire pitch such that the volume of work function settingmaterial is reduced as nanowire pitch decreases, multiple thresholdsnanowire FET devices can be fabricated simultaneously. This is whydevices having different nanowire pitch are shown in the figures so asto illustrate this aspect of the present techniques.

A SOI wafer typically includes a layer of a semiconductor material (alsocommonly referred to as a semiconductor-on-insulator layer or SOI layer)separated from a substrate by an insulator. According to the presenttechniques, the SOI layer will serve as an active layer of the device inwhich the nanowires and pads are patterned. When the insulator is anoxide (e.g., silicon dioxide (SiO₂)), it is commonly referred to as aburied oxide, or BOX. See FIG. 1 wherein nanowires 102 a/102 b and pads104 a/104 b have been patterned in the wafer for the wide pitch andtight pitch nanowire FET devices, respectively. BOX 106 is visiblebeneath the patterned nanowires 102 and pads 104. A substrate istypically located beneath the BOX, but is not visible in the depictionof FIG. 1 (and for ease and clarity of description is not shown in theother figures).

The nanowires and pads (based on a composition of the SOI layer) arepreferably formed from a semiconducting material, such as silicon (Si)(e.g., crystalline silicon), silicon germanium (SiGe) or silicon carbon(SiC). The nanowires and pads may be doped or undoped depending on theparticular device application at hand. By way of example only, asdescribed above, a portion of the nanowires will serve as channels ofthe device(s). When an NFET device is being formed it may be desirableto dope the nanowires with a p-type dopant. When a PFET device is beingformed, it may be desirable to dope the nanowires with an n-type dopant.Suitable p-type dopants include, but are not limited to, boron. Suitablen-type dopants include, but are not limited to, phosphorus and arsenic.Alternatively, the nanowires and pads may be left undoped.

The process for patterning nanowires and pads in a SOI wafer aredescribed in detail in U.S. patent application Ser. No. 13/564,121,filed by Bangsaruntip et al., entitled “Epitaxially Thickened Doped orUndoped Core Nanowire FET Structure and Method for Increasing EffectiveDevice Width” (hereinafter “U.S. patent application Ser. No.13/564,121”), the contents of which are incorporated by referenceherein. As described in U.S. patent application Ser. No. 13/564,121 thenanowires and pads may be patterned using reactive ion etching (RIE)through a hardmask. Since the pattern of the hardmask dictates theconfiguration of the nanowires and pads, in the present example, thepitch of the nanowires may be set by the dimensions of the hardmask. Thesteps for configuring a hardmask for patterning devices with differentnanowire pitch on a wafer would, given the present description, bewithin the capabilities of one skilled in the art.

The devices being fabricated herein are gate all around (GAA) devicesmeaning that the gate(s) being formed will surround a portion of each ofthe nanowires. In order to do so, the nanowires need to be released fromthe underlying substrate (which in this present example is theunderlying BOX 106) in order to expose a surface around each of thenanowires on which the gate(s) can be formed.

The nanowires may be released from the underlying BOX 106 byundercutting the BOX 106 beneath the nanowires using an isotropicetching process using, e.g., a diluted hydrofluoric acid (DHF). A 100:1DHF etches approximately 2 nm to 3 nm of BOX layer 106 per minute atroom temperature. As a result, the nanowires are now suspended over theBOX 106. See FIG. 2, described below.

Further processing, if so desired may now be employed, to re-shapeand/or thin the nanowires. Re-shaping will smoothen the nanowires givingthem an elliptical and in some cases a circular cross-sectional shape.The smoothing of the nanowires may be performed, for example, byannealing the nanowire cores in a hydrogen-containing atmosphere.Exemplary annealing temperatures may be from about 600 degrees Celsius(° C.) to about 1,000° C., and a hydrogen pressure of from about 600torr to about 700 torr may be employed. Exemplary techniques forsuspending and re-shaping nanowires may be found, for example, in U.S.Pat. No. 7,884,004, the contents of which are incorporated by referenceherein. During this smoothing process, the nanowire cores are thinned.According to one exemplary embodiment, the nanowires at this stage havean elliptical cross-sectional shape with a cross-sectional diameter offrom about 7 nm to about 35 nm.

Thinning of the nanowires may be accomplished using a high-temperature(e.g., from about 700° C. to about 1,000° C.) oxidation of the nanowiresfollowed by etching of the grown oxide. The oxidation and etchingprocess may be repeated x number of times to achieve desired nanowiredimensions. According to one exemplary embodiment, the nanowires at thisstage after being further thinned have a cylindrical cross-sectionalshape with a cross-sectional diameter of from about 2 nm to about 20 nm,e.g., from about 3 nm to about 10 nm. Thinning the nanowires serves toincrease the spacing between adjacent nanowires (i.e.,nanowire-to-nanowire spacing).

Next, gate stacks are formed surrounding a portion of each of thenanowires. As highlighted above, the portions of the nanowiressurrounded by the gates will serve as channel regions of the device(s),and portions of the nanowires extending out form the gates and the padswill serve as source and drain regions of the device(s). As alsohighlighted above, and as will be described in detail below, the gatestacks will each contain a work function setting material, an amount ofwhich (by way of the present fabrication process) is proportional to thenanowire pitch of the device. The work function setting material (i)acts as a doping source, and by way of the present process serves tochange the work function of the gate stacks, and (ii) will diffuse intothe surrounding dielectric and gate material to change the thresholdvoltage of the device. To help illustrate the gate fabrication process,the perspective of the figures will now shift to a cross-sectional cutthrough the nanowires, e.g., a cut along line A1-A2—see FIG. 1. Thenanowires in the cross-sectional views are shown enlarged as compared toFIG. 1 in order to better illustrate the various layers of the gatestack deposition.

The first step in the gate stack fabrication process is to form aninterfacial oxide 202 a/202 b surrounding each of the nanowires 102a/102 b, respectively. See FIG. 2. The interfacial oxide prepares thenanowires for the subsequent deposition of a high-k gate dielectric (seebelow). The interfacial oxide will form, for example, by exposing thewafer to an oxygen-containing environment. By way of example only, whenthe nanowires are formed from silicon, the interfacial oxide formed inthis step would contain silicon dioxide (SiO₂). According to anexemplary embodiment, the interfacial oxide is formed to a thicknesst_(io) (see FIG. 2) of from about 0.5 nm to about 3 nm.

A conformal gate dielectric 204 a/204 b is then deposited on theinterfacial oxide 202 a/202 b, respectively, surrounding each of thenanowires. See FIG. 2. According to an exemplary embodiment, the gatedielectric 204 a/204 b is formed from a high-k dielectric material, suchas hafnium oxide, hafnium silicon-oxynitride, or hafniumsilicon-nitride. Suitable conformal deposition processes include, butare not limited to chemical vapor deposition (CVD). By way of exampleonly, the gate dielectric 204 a/204 b is deposited to a thickness t_(gd)(see FIG. 2) of from about 1 nm to about 5 nm.

Further, as highlighted above, the nanowires have been suspended overthe BOX 106 in order to permit GAA devices to be formed (i.e., whereinthe gate fully surrounds at least a portion of each of the nanowire(channels)). As provided above, the nanowires can be suspended byrecessing the BOX 106 beneath the nanowires. This recessed BOX 106 isdepicted in FIG. 2.

Next, as shown in FIG. 3, a conformal first gate material 302 a/302 b isdeposited on the gate dielectric 204 a/204 b, respectively, surroundingeach of the nanowires. According to an exemplary embodiment, the firstgate material consists of a single layer or multiple layers of a gatemetal(s) such as titanium and/or tantalum, e.g., titanium nitride and/ortantalum nitride, and combinations including at least one of theforegoing metals. Suitable deposition processes for conformallydepositing the first gate material (especially in the case of titaniumand tantalum gate metals) include, but are not limited to CVD.

As will be described in detail below, a work function settingmaterial(s) will be deposited onto the first gate material (e.g.,metal(s)). The work function setting material acts as a doping source,and by way of the present process serves to change the work function ofthe gate stacks. Since the work function setting material acts as adoping source, advantageously, the present process flow permits the samegate metal to be used in each of the devices being formed (whichsimplifies the fabrication process). A different work function settingmaterial can then be employed depending, e.g., on whether an n-channelnanowire FET or a p-channel nanowire FET is desired. Further, the gatemetal will diffuse into the surrounding dielectric to change thethreshold voltage of the device. In one exemplary embodiment, the moremetal present in the gate, the lower the threshold voltage (V_(T)) ofthe device. Thus, by adding a work function setting material to the gatestack, the threshold voltages of the resulting devices can be modulated.By way of the present techniques, more of the work function settingmaterial will be deposited in the wide pitch devices as compared to thetight pitch devices. By modulating the volume of work function settingmaterial proportionally to the nanowire pitch such that the volume ofwork function setting material is reduced as nanowire pitch decreases,multiple thresholds nanowire FET devices can be fabricatedsimultaneously.

Accordingly, the volume of the gate material and the volume of the workfunction setting material (to be deposited as described below) areimportant parameters to the present process. According to the presenttechniques, the volume of these materials is quantified based on thethickness of these layers. By way of example only, as shown in FIG. 3,the first gate material 302 a/302 b is deposited to a thickness T_(gm)of (i.e., a uniform thickness across all of the devices) from about 2 nmto about 20 nm.

Next, as shown in FIG. 4, work a function setting material 402 a/402 bis deposited on the first gate material 302 a/302 b, respectively, atleast partially surrounding each of the nanowires. As provided above,the work function setting material acts as a doping source, and adifferent work function setting material can then be employed dependingon whether an n-channel nanowire FET or a p-channel nanowire FET deviceis desired. Thus, the same first gate material 302 a/302 b (e.g.,titanium nitride or tantalum nitride) can be used in each of thedevices, yet a different (if so desired) work function setting materialcan be used in one or more devices to obtain a different dopingpolarity. By way of example only, suitable work function settingmaterials for use in p-channel nanowire FET devices include, but are notlimited to aluminum, dysprosium, gadolinium, and ytterbium. Suitablework function setting materials for use in n-channel nanowire FETdevices include, but are not limited to lanthanum, titanium, andtantalum. Suitable deposition processes for depositing the work functionsetting material(s) include, but are not limited to CVD. ConventionalCVD processes can be tuned to deposit differently on tight pitch andwide pitch nanowires by, for example, operating in a flow-limited regimewhere the flow of reactants and by-products is more restricted in thespace between nanowires at a tight fin pitch. Alternatively, as will bedescribed in detail below, an angled work function metal depositionprocess, such as evaporation or sputtering, may be employed so as tofurther control the amount of work function setting material depositedin proportion to the nanowire pitch.

As also described above, the work function setting material serves tomodulate the threshold voltage (V_(T)) of the device. In one exemplaryembodiment, the work function setting material serves to lower thethreshold voltage (V_(T)) of the device. By way of the present process,the work function setting material(s) can be deposited to a giventhickness (T_(wsm), see FIG. 4) on all devices (e.g., to a thickness offrom about 5 nm to about 20 nm), but because of the configuration of thegate stacks, the volume of work function setting material present ineach gate stack will be proportional to the pitch of the nanowires. Seefor example in FIG. 4 where it is shown that a greater volume of workfunction setting material is present in the wide pitch nanowire FETversus the tight pitch nanowire FET. The present techniques rely onusing the nanowire pitch variation to intentionally vary the amount(volume) of the work function setting material. Therefore, the amount(volume) of the work function setting material is varied by having adifferent size gap for the work function setting material to fill in asa function of nanowire pitch. Namely, as shown in FIG. 4, the workfunction setting material 402 a deposited on the wide pitch nanowire FETsurrounds each of the nanowires, while the work function settingmaterial 402 b deposited on the tight pitch nanowire FET (due to thetight pitch of the nanowires) only surrounds a portion of each of thenanowires (partially surrounds). As a result, the overall amount of thework function setting material surrounding the nanowires in the tightpitch device is less than the amount of the work function settingmaterial surrounding the nanowires in the wide pitch device. The spacingbetween the nanowires (based on the pitch of the nanowires) limits theamount of the work function setting material that can be depositedbetween the nanowires. If so desired, it is not necessary to vary thecomposition of the work function setting material used in the devices,and embodiments are anticipated herein where the same work functionsetting material (deposited, e.g., to the same thickness) is used ineach of the work function setting material layers 402 a and 402 b. It isnotable that nanowires on the ends of the ladder (the end devices),i.e., the first and last nanowires, might get more material since theymight not have a structure adjacent to them to provide a limited gap fordeposition (such as the gap present between the nanowires). However,nanowire FETs generally have several nanowires in parallel (10-20nanowires), so the impact of the first/last nanowire will be lessenedthrough averaging.

As described above, an angled deposition process may be employed todeposit the work function setting material. By way of example only,suitable angled deposition processes include, but are not limited toevaporation or collimated sputtering. By employing an angled depositionprocess, less of the work function setting material will get depositedin the tighter pitch devices. See FIG. 5. Specifically, FIG. 5 whichfollows from FIG. 3 (as an alternate to the deposition process used todeposit the work function setting material in FIG. 4) illustrates howwhen an angled deposition process is used to deposit the work functionsetting material, less of the work function setting material getsdeposited between the tight pitch devices as compared to the wide pitchdevices. The reason for this variable deposition amount is that,depending on the angle of deposition, adjacent gate stacks will “shadow”each other. In this example, with a deposition angle θ of from about 5degrees to about 45 degrees, the gate stacks in the tight pitch deviceswill shadow each other resulting in a lesser amount of the work functionsetting material getting deposited between the gate stacks in thesedevices (as compared to the wide pitch devices).

The remainder of the gate stack which, according to an exemplaryembodiment, consists of a second gate material 602 a/602 b (e.g., withthe conformal gate material 302 a/302 b constituting the first gatematerial) being blanket deposited onto the structure (i.e., over thework function setting material so as to surround the nanowires). Thissecond gate material may be a single layer or may include multiplelayers such as, but not limited to, a layer of material that is the sameas the first gate material, poly silicon, and/or a dielectric cappinglayer such as silicon nitride. For illustrative purposes, the depositionof the second gate material is represented in conjunction with theembodiment where a non-angled deposition of the work function settingmaterial is employed. Thus, the structure shown in FIG. 6 follows fromthat shown in FIG. 4. However, this is merely exemplary, and the secondgate material could in the same manner be formed on the (angle depositedwork function setting material) structure of FIG. 5.

In the exemplary embodiment shown illustrated in FIG. 6, an amount ofthe second gate material deposited in this step is such that each of thenanowires is fully encapsulated (i.e., as opposed to conformallysurrounding each of the nanowires individually as with the first gatematerial). As would be apparent to one of skill in the art, this resultmay be accomplished based simply on the amount of material deposited.Thus, for example, using a deposition process like CVD, the second gatematerial would first form conformally around the nanowires, then as morematerial is deposited, the second gate material will encapsulate andbury the nanowires under a continuous layer of the material. Thus, theresulting structure shown in FIG. 6 can be achieved simply by varyingthe deposition time (and hence the amount of material deposited).

To facilitate illustration of the remainder of the process, theperspective of the figures will now shift to a three-dimensionaldepiction of the device structure. By way of reference to FIG. 7, ahardmask 702 a/702 b (e.g., a nitride hardmask, such as silicon nitride(SiN)) may then be formed on the second gate material 602 a/602 b,respectively, wherein the hardmasks correspond to a gate line of thedevice. Standard patterning techniques can be used to form the hardmask702 a/702 b. The gate material(s), work function setting material anddielectric(s) are then etched by directional etching that results instraight sidewalls of the gate stack 704 a/704 b, as shown in FIG. 7. Anisotropic lateral etch is then performed to remove residue of the gatematerials underneath nanowires, shadowed from the first directionaletching (not shown). This process could be accomplished by RIE or achemical wet etch. After the lateral etching step, formation of thepatterned gate stacks 704 a/704 b over the suspended nanowires iscomplete.

Spacers 802 a/802 b are formed on opposite sides of gate stack 704 a/704b. See FIG. 8. According to an exemplary embodiment, spacers 802 a/802 bare formed by depositing a blanket dielectric film such as siliconnitride and etching the dielectric film from all horizontal surfaces byRIE. As shown in FIG. 8, some of the deposited spacer material canremain in the undercut regions, since the RIE in that region is blockedby the pads.

Next a selective epitaxial material (labeled “Epitaxy”) such as Si,SiGe, or SiC is then grown on the exposed portions of the nanowires andpads (i.e., those portions not covered by a gate stack or spacers) tothicken the exposed portions of the nanowires and pads. See FIG. 9. Thegrowth process might involve epitaxially growing, for example, in-situdoped Si, SiGe or SiC that may be either n-type or p-type doped. By wayof example only, a chemical vapor deposition (CVD) reactor may be usedto perform the epitaxial growth. For example, for silicon epitaxy,precursors include, but are not limited to, SiCl₄, SiH₄ combined withHCL. The use of chlorine allows selective deposition of silicon only onexposed silicon. A precursor for SiGe growth may be GeH₄, which mayobtain deposition selectivity without HCL. Precursors for dopants mayinclude PH₃ or AsH₃ for n-type doping and B₂H₆ for p-type doping.Deposition temperatures may range from about 550° C. to about 1,000° C.for pure silicon deposition, and as low as 300° C. for pure Gedeposition.

Finally, a contact material, in this case a silicide 1002a/1002 b(formed from the epitaxial Si, SiGe or SiC) is formed on the exposedepitaxial material (i.e., the epitaxial material on the pads andportions of the nanowires that extend out from the gate stack). See FIG.10. Examples of contact materials include, but are not limited to,nickel silicide or cobalt silicide. By way of example only, formationtemperatures can be from about 400° C. to about 600° C. The silicideprocess involves reacting a deposited metal(s) (such as nickel and/orcobalt) with silicon (e.g., the epitaxial silicon formed in the previousstep).

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. A nanowire field effect transistor (FET) device,comprising: a semiconductor-on-insulator (SOI) wafer comprising a SOIlayer over a buried oxide (BOX), and a plurality of nanowires and padspatterned in the SOI layer wherein the pads are attached at oppositeends of the nanowires in a ladder-like configuration, and wherein thenanowires are suspended over the BOX, and wherein the nanowirespatterned in the SOI layer have a pitch comprising at least a firstpitch, and at least a second pitch that is different from the firstpitch; an interfacial oxide surrounding each of the nanowires; and gatestacks surrounding the nanowires, such that at least a first one of thegate stacks corresponding to at least one first nanowire FET is formedover the nanowires having the first pitch and at least a second one ofthe gate stacks corresponding to at least one second nanowire FET isformed over the nanowires having the second pitch, each of the gatestacks having (i) a conformal gate dielectric present on the interfacialoxide, surrounding the nanowires (ii) a conformal first gate material onthe conformal gate dielectric, surrounding the nanowires (iii) a workfunction setting material on the conformal first gate material, at leastpartially surrounding the nanowires, and (iv) a second gate material onthe work function setting material, surrounding the nanowires, wherein avolume of the conformal first gate material and a volume of theconformal work function setting material in the gate stacks areproportional to the pitch of the nanowires, wherein the work functionsetting material is configured to change threshold voltages of thedevice, and wherein, by way of the volume of the work function settingmaterial in the gate stacks being proportional to the pitch of thenanowires, the first nanowire FET comprising the first one of the gatestacks formed over the nanowires having the first pitch has a differentthreshold voltage from the second nanowire FET comprising the second oneof the gate stacks formed over the nanowires having the second pitch. 2.The nanowire FET of claim 1, wherein the nanowires and the pads comprisea semiconductor material selected from the group consisting of: silicon,silicon germanium and silicon carbon.
 3. The nanowire FET of claim 1,wherein the interfacial oxide has a thickness of from about 0.5nanometers to about 3 nanometers.
 4. The nanowire FET of claim 1,wherein the conformal gate dielectric comprises a high-k dielectricmaterial.
 5. The nanowire FET of claim 4, wherein the high-k dielectricmaterial is selected from the group consisting of: hafnium oxide,hafnium silicon-oxynitride, and hafnium silicon-nitride.
 6. The nanowireFET of claim 1, wherein the conformal gate dielectric has a thickness offrom about 1 nanometer to about 5 nanometers.
 7. The nanowire FET ofclaim 1, wherein the conformal first gate material comprises a metal. 8.The nanowire FET of claim 7, wherein the metal is selected from thegroup consisting of: titanium, titanium nitride, tantalum, tantalumnitride, and combinations comprising at least one of the foregoingmetals.
 9. The nanowire FET of claim 1, wherein the conformal first gatematerial has a thickness of from about 2 nanometers to about 20nanometers.
 10. The nanowire FET of claim 1, wherein the nanowire FETcomprises a p-channel FET and wherein the work function setting materialcomprises aluminum, dysprosium, gadolinium, or ytterbium.
 11. Thenanowire FET of claim 1, wherein the nanowire FET comprises an n-channelFET and wherein the work function setting material comprises lanthanum,titanium, or tantalum.
 12. The nanowire FET of claim 1, wherein thesecond gate material comprises polysilicon.
 13. The nanowire FET ofclaim 1, further comprising: spacers on opposite sides of the gatestack.
 14. The nanowire FET of claim 13, wherein the spacers comprisesilicon nitride.
 15. The nanowire FET of claim 1, wherein the BOX isundercut beneath the nanowires.
 16. The nanowire FET of claim 1, furthercomprising: an epitaxial material grown on exposed portions of thenanowires and pads.
 17. The nanowire FET of claim 16, furthercomprising: a contact material formed on the epitaxial material.
 18. Thenanowire FET of claim 17, wherein the contact material comprises asilicide.